1. Field of the Invention
The present invention relates to a control device of a servo press and a method for controlling a servo press for driving a slide by a servomotor via, e.g., an eccentric mechanism using a crank shaft and an eccentric shaft and a doubling mechanism using a knuckle mechanism and a linkage mechanism.
2. Related Art of the Invention
FIG. 1 shows a structure of a popular machine press. The energy of a flywheel 3 is transferred from a pinion gear 5 to a main gear 6 via a clutch 4 including fraction plates to drive a crankshaft 8 and to thereby raise or lower a slide 10.
The clutch 4 has a torque transfer capability satisfying the stroke pressure diagram of FIG. 3 showing a pressure force of the press. High torque of the flywheel 3 is transferred to the clutch 4, by which the transfer torque is limited to at or below required torque. Therefore, the clutch 4 does not stress the drive system including the pinion gear 5, the main gear 6, the crankshaft 8, and a connecting rod 9.
FIG. 2 is a structure diagram of a popular servo press, in which the clutch 4 used in the above machine press is not provided and a servomotor 11 is connected to the pinion gear 5 directly or via a deceleration gear. To control the servo press by numeric values such as a position and speed of the slide 10, the rotary position of the servomotor 11 and the position of the slide 10 remain stationary relative to one another, without providing the clutch 4 of the machine press.
As the servomotor 11, a motor having enough high torque to satisfy the stroke pressure diagram of FIG. 3 is selected. Therefore, a pressure force that can be generated by the servo press is shown in the diagram of the servo press of FIG. 3. In FIG. 3, the horizontal axis shows a height (the unit is mm) from the bottom dead center of the slide and the vertical axis shows a pressure force (the unit is kN) of the press.
However, the servomotor 11 can generate sufficient torque, but when excessive torque is applied in the structural design of the press machine, the drive system including the pinion gear 5 and main gear 6 is damaged, e.g., to pitch. Then, when continuously used, the press machine may be broken.
Past overload protection devices for presses include: a protection device having an oil pressure chamber provided to the connection portion of the connecting rod 9 and slide 10 to leak oil pressure when a pressure force at or over a preset pressure force is generated; and an overload detection device having a strain gauge attached to a frame of a press machine to detect a pressure force as extension of the frame and to stop the press when detecting a predetermined value or over.
However, a speed of the servomotor 11 is variable also during pressing. The torque transferred from the servomotor 11 to a drive shaft changes between the acceleration and deceleration of the servomotor 11 even when the pressing load does not change. The overload protection devices of past machine presses cannot protect the presses.
As a measure for this disadvantage in JP-A No. 2004-174591, to protect a machine of a servo press, motor torque is decreased in response to an eccentric shaft rotation angle to keep a limitation value of a slide pressure force generally constant near the slide bottom dead center.